It is known that hydrophobic resin sheet and web materials of low surface conductivity readily become electrostatically charged due to friction with dielectric materials and triboelectrically chargeable transport means, such as, rollers. The electrostatic charging is particularly severe in relatively dry environments and at high speeds of conveyance. An electrostatically charged web can result in static discharge through generation of sparks which poses fire hazards in the presence of flammable solvents at a typical coating site. For a web containing an unprocessed photographic recording element, sparking can cause additional problems, such as, developable fog and degradation of the image quality. So, it is very important to provide antistatic protection on photographic films and paper.
For photographic paper, an additional criterion is the ability of the antistatic backing layer to receive printing (e.g., bar codes or other indicia containing useful information) typically administered by dot matrix or inkjet printers and to retain these prints or markings as the paper undergoes processing (viz, backmark retention).
From U.S. Pat. No. 3,525,621, it is known that antistatic properties can be given to an aqueous coating composition by practically any silica sol, but preferably silica particles of large surface area of the order of 200-235 m.sup.2 /g in combination with an alkylaryl polyether sulphonate. However, the high solubility of the alkylaryl polyether sulphonate in aqueous medium will cause leaching during processing resulting in poor backmark retention.
Antistatic layers on the basis of solely colloidal silica usually show microcracks upon drying which can lower the lateral conductivity. Additionally calcium stearate from the paper base often leaches out through these cracks causing stearate sludge in the processing tanks, requiring costly clean up operations.
U.S. Pat. No. 4,173,480 teaches the use of synthetic hectorite as antistatic additive to a silica containing layer. However, the hydrophilicity of the hectorite results in poor backmark retention upon exposure to processing solutions. In fact, the experience in the trade is most colloidal silica based antistatic backings without a polymeric binder provide poor post-processing backmark retention qualities for photographic paper.
U.S. Pat. No. 5,244,728 discloses backing formulations containing alumina modified colloidal silica and an antistatic agent in a polymeric binder consisting of an addition product of alkyl methacrylate, alkali metal salt and vinyl benzene. Although such backing layers provide adequate antistatic protection and backmark retention characteristics, these layers lack sufficient mechanical integrity as manifested in poor spliceability and track off characteristics. Heat splicing of photographic paper rolls is often carried out during printing operations and is expected to provide enough mechanical strength to resist peeling as the web goes through automatic photographic processing. Poor splice strength can cause a number of problems including jamming of automatic processing equipment. Track off during conveyance can lead to undesirable build-up of materials on conveyance rollers and other surfaces often causing product defects.
An object of the present invention is to provide an antistatic backing layer on an imaging element containing paper support, specifically photographic paper, with backmark receiving and retention qualities, heat spliceability and good trackoff characteristics.
While the invention herein finds particular use in the photofinishing industry to print barcodes or other indicia on the back of paper prints by using dot matrix printers for example, it is useful and suitable for applying print or ink markings to any surface wherein the original surface does not possess the desired characteristics. The application with regard to photofinishing has a particularly stringent requirement because the backing layer must survive photographic processing through the automatic processing devices having the harshest conditions in order to be useful.
In photofinishing applications, the coating compositions must satisfy the following requirements:
1. The ingredients must be compatible. This is a particularly stringent requirement when antistatic agents are formulated in the coating composition so that the print retaining layers also possess antistatic properties. The polymeric binder in the coating composition in the form of a latex can be easily destabilized causing agglomeration of the latex particles to occur.
2. The coatings must be alkali resistant up to a pH of 10 to survive the photographic processing solutions.
3. The coatings must be resistant to discoloration due to processing solutions and/or aging.
4. The coatings must be able to receive and retain ink or other marking materials through the photographic processing.
5. The coatings must not be photoactive and interfere with the light sensitive portions of the photographic paper.
6. The coatings must have resistivity less than 12 log ohms per square at 50% RH.
7. The backside coating must be spliceable to the frontside in commercially available splicing devices and maintain sufficient peel strength.
8. The coatings must be resistant to track off during conveyance by various roller/nip transport machines during manufacturing of the photographic paper and also in the development processor.
9. The coatings must be block resistant in the rolled form. That is, in preparation of printing paper for use in photographic applications, the paper in processing is rolled upon itself. It is necessary that the backmark retaining layer does not block or ferrotype together with the opposite surface of the paper support.
10. The coatings must have a stability of from 6 to 12 months in order to be commercially acceptable.
The coatings and the coating compositions according to this invention satisfy these requirements by utilizing in combination an electrically conducting synthetic smectite clay and a polymeric binder.